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In the fabrication of printed circuit boards, a thin coating of tin, typically 0.1 microns in thickness, is applied to a copper or copper alloy substrate. A silane coupling agent is applied over the tin coating. This process is primarily used as a bonding enhancement to improve the subsequent adhesion of an epoxy prepreg to the metal of a multilayer printed circuit board. This tin coating produces only marginal improvement as a bonding intermediate for the adhesion of copper to no-flow and low-flow epoxy prepregs. Also, the tin coating is typically too thin and porous to be used as an etch resist for the manufacture of printed circuit boards.
Examples of tin coating processes for circuit boards may be found in U.S. Pat. Nos. 5,073,456, 5,196,053, 5,211,831, and 5,217,751.
The present invention provides the use of multiple tin coatings that have superior chemical resistance and enhance adhesion in fabricating printed circuit boards. A first coating of tin provides a white tin layer that serves as an etch resist at the primary or secondary etching operation during the fabrication of the printed circuit traces. A second coating of tin provides a layer that serves as a bonding agent or bonding enhancement layer that allows the lamination of no-flow and low-flow prepreg materials to a copper surface with resulting adhesion between the polymer and the metal exceeding four pounds per inch.